A fiber Bragg Grating (FBG) is a periodic modulation of the refractive index of the core of an optical fibre. The index modulation is induced by exposing the fibre to an interference pattern formed between interfering ultraviolet (UV) beams of light. The resulting grating acts as a reflective (or transmission) wavelength filter. The reflected wavelength (λB), called Bragg wavelength, is defined by the relationship:
λB=2nΛ
where n is the effective refractive index of the core and Λ is the grating period. The working principle of the FBG is shown in the following picture.
As well as being sensitive to strain, the Bragg wavelength is also sensitive to temperature. This means that fiber Bragg gratings can be used as sensing elements. In a FBG sensor, the measurand causes a shift in the Bragg wavelength, ΔλB. The relative shift in the Bragg wavelength, ΔλB/λB, due to an applied strain (ε) and a change in temperature (ΔT) is approximately given by:
Where pe is the strain optic coefficient, αΛ is the thermal expansion coefficient of the optical fiber and αn is the thermo-optic coefficient.
Fiber Bragg gratings can then be used as direct sensing elements for strain and temperature. They can also be used as transduction elements, converting the output of another sensor, which generates a strain or temperature change from the measurand.